The present invention relates generally to microelectronic devices, and more particularly to microelectronic devices encapsulated in barrier stacks.
Microelectronic devices fabricated on semiconductor substrates require passivation, or encapsulation, to protect them from atmospheric contaminants and constituents, mechanical damage, stress, thermal stress and cycling, downstream processing, and corrosive chemicals.
Passivation of the microelectronic devices performs several functions. First, it electrically insulates the microelectronic device from other microelectronic devices. It preserves the recombination velocity at the semiconductor surface. It is also a stress buffer to minimize cracking. It provides protection from processing chemicals, ultraviolet light exposure, and photoresists during lithography processes. In addition, it provides protection from humidity, oxidants, corrosive materials, scratching, and mechanical damage. Finally, it provides gettering of mobile ions, such as Clxe2x88x92, and Na+. Lavinger et al., J. Vac. Sci. Technol. A16(2), Mar./Apr. 1998, p.530.
Conventional hermetic sealing in metal or ceramic provides effective protection. However, conventional hermetic enclosures are relatively bulky (about 4-6 mm deep), and they add a significant amount of weight to the product, which reduces the benefits of miniaturization.
Many devices that require passivation are now fabricated on glass, fused silica, and ceramic substrates. For example, thin films of amorphous silicon nitride (Si3N4) and silicon dioxide (SiO2) are used on p-type semiconductor devices as a protective coating, a mask for lithography processes, charge storage systems in nonvolatile metal-nitride-oxide-semiconductor memory devices, insulators between metal layers, gate insulators for thin film transistors, and ultrathin dielectrics for very large scale integration devices. For integrated circuit applications, SiO2 is the stress buffer layer and Si3N4 is the passivation layer, as described in U.S. Pat. No. 5,851,603, which is incorporated herein by reference. Silicon nitride is primarily deposited by chemical vapor deposition processes (CVD), such as atmospheric pressure chemical vapor deposition (APCVD), low pressure chemical vapor deposition (LPCVD), and plasma enhanced chemical vapor deposition (PECVD). However, the use of inorganic materials deposited by standard semiconductor processes such as chemical vapor deposition has several disadvantages. The most serious disadvantages are brittleness, a tendency to crack under mechanical stress, poor step coverage, poor planarization properties, and poor barrier properties. The best oxygen permeation rate for Si3N4 deposited by electron cyclotron resonance-plasma enhanced vapor deposition (ECR PECVD) is reported to be near 1 cc/m2/day. The best oxygen permeation rate for SiO2 is also near 1 cc/m2/day.
As a result of these problems, there has been a significant effort to replace inorganic materials such as SiO2 and Si3N4 with polymer dielectrics. Polymer materials of interest include polyimide, polyamide, and paralyene. Organic materials offer good adhesion, sufficient elasticity, and sufficient tensile strength. However, these materials have problems with brittleness and defects such as voids. C. P. Wong, Ceramic Trans. 33, 1993 p. 125.
The barrier protection offered by inorganic and organic materials is not usually adequate to ensure reliable microelectronic device operation. Additional barrier layers are added prior to encapsulation. Materials such as silicon rubber are used as barriers. The integrated circuit can be embedded in plastic by injection molding to add further moisture barrier protection.
Epoxies are also used for barrier applications and encapsulation. The epoxy layers used for encapsulation are only about one quarter of the thickness of the layers required for convention hermetic sealing. However, even that thickness produces a device which is unacceptably heavy and bulky in many applications. In addition, epoxies have a water vapor permeation rate which is too high for some applications.
The passivation layers currently being used in microelectronics include silicon dioxide, silicon nitride, and silicon oxynitride layers with thicknesses up to about 1 xcexcm. These layers are deposited by CVD and reactive magnetron sputtering processes, which can require substrate and processing temperatures as high as 800xc2x0 C. Materials deposited by CVD can also have very stresses (i.e., greater than 10,000 MPa).
The inorganic layer is often followed by a spin cast polyimide layer about 0.5 xcexcm thick. The polyimide layer is used for passivation, encapsulation, planarization, and bonding/molding to the packaging. The layer is spun on and cured at temperatures up to 250xc2x0 C. The oxygen and water vapor barrier properties of polyimide are poor and typical of polymer substrates ( greater than 10 cc/m2/day). Polyimide is very opaque and strongly absorbing at visible wavelengths. Polyimide films can have large numbers of voids, which can cause reliability problems with integrated circuits. The voids can also cause hot spots and cracks that can damage integrated circuit components.
Another method used to protect microelectronic circuitry is vapor deposition of a thin film of parylene. However, the water vapor permeation rate of the parylene is too high for many applications. In addition, parylene is subject to thermal oxidation at temperatures over about 120xc2x0 C.
Furthermore, PECVD coatings have problems with pinholes, poor step coverage, and particulates. The quality of the coating is usually poor. Deposition processes for these layers can damage temperature sensitive material in, for example, integrated circuits, organic light emitting devices, light emitting polymers and microlasers. As a result, totally effective encapsulation of temperature sensitive devices cannot be achieved on semiconductor substrates using conventional deposition processes. Additionally, in order to obtain the required encapsulation and passivation, the current passivation layers must be thick compared to device thicknesses and sizes, which causes problems in the fabrication of multilevel integrated circuits. Finally, as discussed above, the barrier properties for these materials are inadequate for many applications.
Thus, there is a need for an improved, lightweight, thin film, barrier construction which can be used to encapsulate microelectronic devices, and for methods for making such encapsulated microelectronic devices.
The present invention meets these needs by providing an encapsulated microelectronic device and a method for making such a device. The device includes a semiconductor substrate, a microelectronic device adjacent to the semiconductor substrate, and at least one first barrier stack adjacent to the microelectronic device. By adjacent, we mean next to, but not necessarily directly next to. There can be additional intervening layers. The barrier stack encapsulates the microelectronic device. It includes at least one first barrier layer and at least one first polymer layer. The encapsulated microelectronic device optionally includes at least one second barrier stack located between the semiconductor substrate and the microelectronic device. The second barrier stack includes at least one second barrier layer and at least one second polymer layer.
Preferably, either one or both of the first and second barrier layers of the first and second barrier stacks is substantially transparent. At least one of the first barrier layers preferably comprises a material selected from metal oxides, metal nitrides, metal carbides, metal oxynitrides, metal oxyborides, and combinations thereof.
Either one of the first and second barrier layers can be substantially opaque, if desired. The opaque barrier layers are preferably selected from opaque metals, opaque polymers, opaque ceramics, and opaque cermets.
The polymer layers of the first and second barrier stacks are preferably acrylate-containing polymers. As used herein, the term acrylate-containing polymers includes acrylate-containing polymers, methacrylate-containing polymers, and combinations thereof The polymer layers in the first and/or the second barrier stacks can be the same or different.
The semiconductor substrate can either be flexible or rigid.
The microelectronic device is preferably selected from integrated circuits, charge coupled devices, light emitting diodes, light emitting polymers, organic light emitting devices, metal sensor pads, micro-disk lasers, electrochromic devices, photochromic devices, microelectromechanical systems, and solar cells.
The encapsulated microelectronic device can include additional layers if desired, such as polymer smoothing layers, scratch resistant layers, or other functional layers. The encapsulated microelectronic device can also include a lid adjacent to the at least one first barrier stack.
The present invention also involves a method of making the encapsulated microelectronic device. The method includes providing a semiconductor substrate having an microelectronic device thereon, and placing at least one first barrier stack over the microelectronic device to encapsulate the microelectronic device. The barrier stack includes at least one first barrier layer and at least one first polymer layer.
The microelectronic device can be placed on the semiconductor substrate by diffusion, ion implantation on deposition, or lamination. The at least one first barrier stack can be placed over the microelectronic device by deposition, preferably vacuum deposition, or by laminating the barrier stack over the environmentally sensitive device. The lamination can be performed using an adhesive, solder, ultrasonic welding, pressure, or heat.
A second barrier stack can be placed on the semiconductor substrate before the microelectronic device is placed there. The second barrier stack includes at least one second barrier layer and at least one second polymer layer. The second barrier stack can be deposited on the semiconductor substrate, preferably by vacuum deposition.
The semiconductor substrate can be removed from the encapsulated microelectronic device, if desired.
Accordingly, it is an object of the present invention to provide an encapsulated microelectronic device, and to provide a method of making such as device.